In response to the recent concern for saving resources or protecting the global environment, recycling of waste paper (old paper) is a social issue and the papermaking industry considers it extremely important to perform enhanced processing of waste paper and expand the use of the recycled paper. In one way to meet this need, the printing ink on the recovered paper is removed by the deinking process to make pulp with a smaller content of residual ink, which is optionally mixed with virgin pulp to make fresh paper.
However, the making of deinked pulp involves one problem with the pulping treatment and the ink detaching treatment; since high shear force is commonly applied under a highly alkaline condition with a pH exceeding 10, the contaminants in the waste paper such as a pressure-sensitive adhesive, a bonding agent, adhesive tape, spine glue on magazines, and vinyl tape become so small in size that it is very difficult to remove them completely even if the deinked pulp is subjected to the subsequent fine screening process as by a screen or a cleaner. If the deinked pulp contains such micronized contaminant, papermaking from that deinked pulp will suffer, for example, a runability problems such as sheet breaks or a deterioration in paper quality due to contamination of the paper with the sticky contaminants. The exertion of high shear force under the alkaline condition causes other difficulties, such as an increased chemical oxygen demand (COD) in the effluent, damaged pulp fibers that have a lower water freeness which results in less efficient paper machine runability, and the formation of pulp that has lower bulk and which is poor in optical characteristics.
Japan TAPPI Journal vol. 49(1), 121-130 (Non-Patent Document 1) proposes a method in which recovered paper is pulped at low temperature under a neutral condition to prevent stickies from becoming undesirably small in size and, after removing the stickies with a screen or by other suitable means as it remains large enough, ink is detached under a highly alkaline condition. This method is anticipated to provide some improvement as regards the stickies but, on the other hand, since the pulp is subjected to high shear force under the highly alkaline condition, there is no anticipation for lower COD.
To deal with these problems, a method has been proposed in which the use of alkali chemicals in the making of deinked pulp (DIP) is sufficiently reduced to establish a neutral condition, whereby the stickies present in waste paper are prevented from becoming unduly small in size and, what is more, the COD in the effluent is reduced. However, the reduced use of alkali chemicals lowers the detachability of ink, eventually deteriorating the quality of the pulp as the final product. To cope with this problem, the official gazette of JP 11-200269 A (Patent Document 1) proposes a method in which recovered paper swelled under an alkaline condition is neutralized with an added acid and after neutralization or simultaneously with it, high shear force is applied, whereby DIP with a smaller content of residual ink is produced while reducing the COD of effluents from the DIP production process. However, the use of an acid to neutralize the alkali-swelled waste paper adds to the overall cost.
The official gazette of U.S. Pat. No. 3,260,211 (Patent Document 2) shows a crystalline silicate having a deinking capability. However, this document does not show using the silicate in a weakly alkaline to neutral region, or a condition where deinking is generally difficult to perform.    Patent Document 1: Official gazette of JP 11-200269 A    Patent Document 2: Official gazette of U.S. Pat. No. 3,260,211    Non-Patent Document 1: Japan TAPPI Journal vol. 49(1), 121-130